Absence duration is correct but deduction from the balance is incorrect

Summary

Deducting extra days for weekoffs because of conversion formula but duration is calculated correctly

Content

UOM of absence  type/plan is Days but we need to include the off days (like saturday /sunday) also in the leave duration. So we added a conversion formula at type using simple FF 

days = DAYS_BETWEEN(IV_END_DATE,IV_START_DATE) +1

duration = round(days)

After adding conversion formula , the duration came correctly eg leave applied from Friday till Monday , the duration shows 4 days which initially was showing 2 days.

The issue is that, from the balance 6 days were deducted ie for weekoffs (saturday/sunday) it deducted 2days for a day so total 6(1+2+2+1).